Yes, that would be possible to use an additional object with a transparent texture.

Now the question is could you add some extra features to the mouse_ent function so that it reacts on the BoundingBox when for example the BBOX-flag is set?
In this case I would wait until this feature is implemented (unless it would take too long :-) ). Otherwise I would have to use the workaround mentioned above.

Regards,
Pegamode